1. Field of the Invention
The present invention relates to a security system which monitors intrusion by a person, an object, or the like.
2. Related Art
Examples of a sensor for monitoring surroundings of a building to detect an intruder include a security system which uses infrared rays. In this system, both of a transmitting unit for transmitting infrared rays and a receiving unit for receiving the infrared rays are installed in an intrusion monitoring area in order to detect interruption of light receiving, and thereby an intruder is detected. An infrared sensor is easily influenced by weather, and it takes time to adjust optical axes of the transmitting and receiving units. In addition, the infrared sensors should be installed in surroundings of the building, which produces a problem of high wiring cost.
In addition, examples of a sensor for detecting an intruder, which uses a radar, include an intruder detecting device disclosed in Japanese Patent Laid-Open No. 2000-3478. This device is capable of knowing the existence, position, number, moving direction of a person by the following steps: transmitting and receiving a radio wave; performing frequency spectrum operation of a state of surroundings; and thereby detecting its change. As described above, because the intruder detecting device which uses a radio wave has a narrow emitting angle of a radio wave, it is difficult to emit a radio wave over a wide area if the intruder detecting device is used outdoors.